Changes in ruminal volatile fatty acid production and absorption rate during the dry period and early lactation as affected by rate of increase of concentrate allowance

he aim of the present experiment was to study changes in volatile fatty acid (VIA) production using an isotope dilution technique, and changes in VIA fractional absorption rate (k(a)VFA) using a buffer incubation technique (BIT) during the dry period and early lactation, as affected by the postpartum (pp) rate of increase of concentrate allowance. The current results are complementary to previously reported changes on rumen papillae morphology from the same experiment. From 50 d antepartum to 80 d pp, VIA production rate was measured 5 times and k(a)VFA was measured 10 times in 12 rumen-cannulated Holstein Friesian cows. Cows had free access to a mixed ration, consisting of grass and corn silage, soybean meal, and (dry period only) chopped straw. Treatment consisted of either a rapid (RAP; 1.0 kg of DM/d; n = 6) or gradual (GRAD; 0.25 kg of DM/d; n 6) increase of concentrate allowance (up to 10.9 kg of DM/d), starting at 4 d pp, aimed at creating a contrast in rumen-fermentable organic matter intake. For the BIT, rumen contents were evacuated, the rumen washed, and a standardized buffer fluid introduced [120 mM VFA, 60% acetic (Ac), 25% propionic (Pr), and 15% butyric (Hu) acid; pH 5.9 and Co-EDTA as fluid passage marker]. For the isotope dilution technique, a pulse-dose of C-13-labeled Ac, Pr, and Hu and Co-EDTA as fluid passage marker was infused. The rate of total VIA production was similar between treatments and was 2 times higher during the lactation (114 mol/d) than the dry period (53 mol/d). Although papillae surface area at 16, 30, and 44 d pp was greater in RAP than GRAD, Bu and Ac production at these days did not differ between RAP and GRAD, whereas at 16 d pp RAP produced more Pr than GRAD. These results provide little support for the particular proliferative effects of Bu on papillae surface area. Similar to developments in papillae surface area in the dry period and early lactation, the k(a)VFA (per hour), measured using the BIT, decreased from 0.45 (Ac), 0.53 (Pr) and 0.56 (Bu) at 50 d ante parturn to 0.28 (Ac), 0.34 (Pr) and 0.38 (Bu) at 3 d pp. Thereafter, k(a)VFA (/h) rapidly increased up to 0.07 (Ac), 0.79 (Pr), and 0.79 (Bu) at 80 d pp. Although papillae surface area was greater at 16, 30, and 44 d pp in RAP titan GRAD, no differences in k(a)VFA between RAP and GRAD were observed during these clays showing papillae surface area is riot the limiting factor for k(a)VFA during early pp adaptation.